RESEARCH ARTICLE Structure of the Collision Zone Between the Nazca Ridge 10.1029/2019TC005637 and the Peruvian Convergent Margin: Geodynamic Key Points: • The Nazca Ridge hosts an and Seismotectonic Implications overthickened lower crust E. Contreras‐Reyes1 , P. Muñoz‐Linford2, V. Cortés‐Rivas1, J. P. Bello‐González3, J. A. Ruiz1, (10–14 km) formed in an on‐ridge 4 setting (hot spot plume near a and A. Krabbenhoeft spreading center) 1 2 • The Nazca Ridge correlates with a Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile, Centro prominent continental slope scarp I‐MAR, Universidad de los Lagos, Puerto Montt, Chile, 3Grupo Minero Las Cenizas, Taltal, Chile, 4GEOMAR‐Helmholtz bounded by a narrow and uplifted Centre for Ocean Research, Kiel, Germany continental shelf • The Nazca Ridge has behaved as a seismic asperity for moderate earthquakes (e.g., 1996 Mw 7.7 and Abstract We study the structure and tectonics of the collision zone between the Nazca Ridge (NR) and 2011 Mw 6.9) nucleating at depths the Peruvian margin constrained by seismic, gravimetric, bathymetric, and natural seismological data. >20 km The NR was formed in an on‐ridge setting, and it is characterized by a smooth and broad shallow seafloor (swell) with an estimated buoyancy flux of ~7 Mg/s. The seismic results show that the NR hosts an Supporting Information: – – • Supporting Information S1 oceanic lower crust 10 14 km thick with velocities of 7.2 7.5 km/s suggesting intrusion of magmatic material from the hot spot plume to the oceanic plate. Our results show evidence for subduction erosion in the frontal part of the margin likely enhanced by the collision of the NR. The ridge‐trench collision zone Correspondence to: correlates with the presence of a prominent normal scarp, a narrow continental slope, and (uplifted) shelf. E. Contreras‐Reyes, In contrast, adjacent of the collision zone, the slope does not present a topographic scarp and the continental [email protected] slope and shelf become wider and deeper. Geophysical and geodetic evidence indicate that the collision zone is characterized by low seismic coupling at the plate interface. This is consistent with vigorous Citation: subduction erosion enhanced by the subducting NR causing abrasion and increase of fluid pore pressure Contreras‐Reyes, E., Muñoz‐Linford, P., Cortés‐Rivas, V., Bello‐González, J. P., at the interplate contact. Furthermore, the NR has behaved as a barrier for rupture propagation of Ruiz, J. A., & Krabbenhoeft, A. (2019). megathrust earthquakes (e.g., 1746 Mw 8.6 and 1942 Mw 8.1 events). In contrast, for moderate earthquakes Structure of the collision zone between (e.g., 1996 Mw 7.7 and 2011 Mw 6.9 events), the NR has behaved as a seismic asperity nucleating at the Nazca Ridge and the Peruvian convergent margin: Geodynamic and depths >20 km. seismotectonic implications. Tectonics, 38, 3416–3435. https://doi.org/10.1029/ 2019TC005637 1. Introduction Received 25 APR 2019 Accepted 21 AUG 2019 Hot spot tracks and oceanic plateaus entering a subduction zone modify the structure of the forearc system Accepted article online 24 AUG 2019 and influence dramatically the coupling degree across the subduction interface affecting both interplate and Published online 11 SEP 2019 intraplate seismicity (e.g., Bilek, 2010; Contreras‐Reyes & Carrizo, 2011; Wang & Bilek, 2011). In addition, the collision of oceanic high features with a convergent margin enhances subduction erosion (e.g., Balance et al., 1989; Hampel et al., 2004; Ranero & von Huene, 2000) and usually modifies locally the morphology of the upper plate causing uplift in the frontal part of the collision zone followed by subsidence as the aftermath of seamount subduction (e.g., Dominguez et al., 1998; Zeumann & Hampel, 2015). As high oceanic features collide with the overriding plate, they will affect differently the coupling degree at the subduction interface and the upper plate deformation depending on the crustal buoyancy mode (Watts et al., 2010). For example, seamounts formed near a spreading center on weak oceanic plate (on‐ridge seamounts like the Cocos and Nazca ridges) are locally more compensated and more buoyant (Watts et al., 2010), and thus, an important coupling at the subduction interface is expected. In addition, these ocea- nic ridges are characterized by a smooth and broad morphology (Orellana‐Rovirosa & Richards, 2017) and more basal subduction erosion beneath the upper plate rather than frontal subduction erosion at the front of the upper plate is hypothesized (Contreras‐Reyes & Carrizo, 2011). In contrast, oceanic ridges formed far from oceanic spreading center (off‐ridge seamounts like the Juan Fernandez and Louisville ridges) are more regionally compensated, and therefore less buoyant (Watts et al., 2010), and host a rough and narrow morphology (Orellana‐Rovirosa & Richards, 2017). Thus, heterogeneous and/or weaker coupling at the ©2019. American Geophysical Union. subduction contact (Wang & Bilek, 2011; Watts et al., 2010) and more frontal subduction erosion are All Rights Reserved. expected to occur (Dominguez et al., 1998; Ranero & von Huene, 2000). CONTRERAS‐REYES ET AL. 3416 Tectonics 10.1029/2019TC005637 The Nazca Ridge (NR) is an on‐ridge hot spot track that was formed together with the Easter Seamount Chain by the interaction between the oceanic Nazca Plate and the Easter/Sala y Gomez hot spot plume posi- tioned near the East Pacific Rise (e.g., Ray et al., 2012). The NR is composed of abnormally thick basaltic oceanic crust, averaging 18 ± 3 km thick (Hampel et al., 2004; Woods & Okal, 1994), and it has a shallow seafloor swell surface about 200–300 km wide that extends for more than 1,100 km across the oceanic Nazca Plate in the N42°E direction (Hampel, 2002). Contreras‐Reyes and Carrizo (2011) estimated an anom- alous normal stress of ~50 MPa associated with the overthickened oceanic crust of the NR, which results in a considerable buoyancy force affecting the initial angle of subduction of the downgoing plate and the subduc- tion erosion rate at the frontal part of the Peruvian convergent margin. For instance, the subduction of the NR has been correlated with a gap in arc volcanism and a subhorizontal subduction of the oceanic Nazca Plate beneath Peru (Gutscher et al., 2000; Espurt et al., 2008). In addition, Clift et al. (2003) and Stern (2011) estimated a long‐term subduction erosion rate of 37–78 km3/km/Myr for the Peruvian margin. These authors, however, pointed out that subduction erosion was highly accelerated during the collision of the NR with the South American Plate with a short‐term subduction rate of 115–228 km3/km/Myr. Little seismic activity of large earthquakes (Mw > 8) in southern Peru (14–15.5°S) has been recorded (Beck & Ruff, 1989; Villegas‐Lanza et al., 2016) along the NR subduction beneath the South American Plate. For instance, there is no seismological record of any large earthquake in the ridge‐trench collision zone since the historic 1868 Mw 8.5–9.0 megathrust earthquake (Villegas‐Lanza et al., 2016). Okal et al. (2006) esti- mated a coseismic slip model for this event characterized by rupture south of the NR, the megathrust above the subducting NR, and at least a small segment of the interplate contact north of the NR. However, these authors used a simple rectangular fault model with uniform slip and their data are based on historic reports with large uncertainties. Nevertheless, moderate thrust events (Mw < 8) such as the 1996 Mw 7.7 Nazca and 2011 Mw 6.9 earthquakes have been attributed to the high friction associated with the buoyant NR (e.g., Spence et al., 1999) highlighting the complex seismic pattern of this subducting oceanic ridge. The 12 November 1996 Mw 7.7 event broke a large zone of the shallow subduction interface of the NR, and the rup- ture extended for about 200‐km length (e.g., Salichon et al., 2003; Spence et al., 1999). On the other hand, the 28 October 2011 Mw 6.9 event, a moderate thrust earthquake occurred at the northern edge of the NR with a centroid depth of 30 km according to the National Earthquake Information Center (NEIC) catalog. Apparently, the NR behaves as a barrier for earthquake rupture propagation of some large events (Mw > 8) and as a seismic asperity for smaller events. This dual seismic behavior of the NR is intriguing and can be partially related to anomalous density zones hosted in the oceanic crust and/or upper mantle causing a heterogeneous buoyancy force transferred to the interplate boundary as the NR subducts. In this work, we used a joint refraction and reflection traveltime tomographic approach to derive a detailed 2‐D velocity‐depth model constrained by gravimetric and bathymetric data. A single seismic profile across the NR in southern Peru includes the outer rise region, trench axis, and the marine forearc (Figures 1 and 2). The profile is located near the epicentral area of the 1996 Mw = 7.7 and 2011 Mw = 6.9 earthquakes. In addition, we provide coseismic slip models for these events, which are used to discuss the role of the NR as barrier and/or asperity for earthquake rupture propagation. We present a general picture of the NR‐trench collision zone and we discuss its geodynamic and seismotectonic implications. 2. Tectonic Setting 2.1. Geodynamic Setting The Peruvian convergent margin is characterized by the subduction of the oceanic Nazca Plate beneath the continental overriding South American Plate, where geodetic observations of the Nazca‐South America convergence rate predict a current azimuth of approximately N78°E direction and a convergence rate of 68–77 mm/year (DeMets et al., 1990; Norabuena et al., 1998). On the other hand, long‐term paleoreconstruc- tion models reported a convergence rate of 88–96 mm/year with an azimuth of ~N79°E (Bello‐González et al., 2018; Müller et al., 2016; Figure 3).